A number of people die of thromboembolic diseases which usually occur in association with conditions such as artherosclerosis, trauma, cancer and premature separation of placenta. Therefore, an effective preventive measure is of utmost importance. The factors responsible for the onset of thromboembolic diseases are at least several, but one of the most important is tissue injury including vascular injury. When there is a tissue injury, one might reasonably expect that the tissue juice including tissue thromboplastin might enter the blood circulation. Thus, tissue thromboplastin might be regarded as being directly responsible for at least some forms of thromboembolic diseases. We therefore propose to purify tissue thromboplastins from brain, lung, artery and placenta and to develop a sensitive method for the measurement of tissue thromoboplastins in blood. BIBLIOGRAPHIC REFERENCES: Gonmori, H. and Takeda, Y. Properties of canine tissue thromoboplastins from brain, lung, arteries and veins. Am. J. Physiol. 229: 618, 1975. Takeda, Y. and Gonmori, H. Canine Tissue Thromboplastins. Physiologist. 18: 416, 1975 (Abstract).